A need for 360 degree propagation of sound energy by a sound reproduction system is essential if realistic reproduction of live sound propagation is to be effected. Virtually all live sources of music propagate sound as if a pebble were dropped in a three-dimensional pool. Many instruments and even human voices propagate with a greater intensity in the forward direction than in other directions. However, energy contributing to quality and loudness is propagated in all directions and arrives at a listener as reflections. The phase relationships of these direct and reflected sound waves allow a listener to locate a sound source respective its location. Many manufactures have attempted 360 degree propagation in a horizontal plane; the need for vertical 360 degree dispersion has not been demonstrated. Some have attempted 360 degree propagation by aiming a plurality of speakers in a plurality of directions about a circle. Some systems have placed all but one speaker facing backwards in order to reflect sound energy from a back wall, thereby attemping to achieve a 360 degree effect. In all of these systems, sound from each source is initiated out-of-phase with respect to sound from the other sources due to the physical displacement of each speaker with respect to the other. All sound waves initiated by a live sound source are by definition in phase. It is this phase relationship that a good sound system tries to accurately reproduce. One way of accomplishing this is to effect 360 degree, in-phase propagation at a sound source, but in practice the desired high level of realism is not achieved.
A speaker system of the present invention provides a 360 degree propagation which does achieve a high level of realism by utilizing any of various types of active reflectors.
In one aspect of the invention, a first dome-type speaker having a longitudinal axis is positioned so that its longitudinal axis is normal to a surface spaced so as to provide an active reflector for sound propagated by the first speaker at a point touching the surface to less than two inches spaced therefrom. The spacing limitations respecting the active reflector are critical to both vertical dispersion and sound pressure level as well as horizontal dispersion. Within the spacing constraint of two inches, a dramatic increase in sound pressure level and excellent horizontal and vertical dispersion is observed at distances corresponding to one-quarter wavelength of the input sine wave, but even at touching (or even at a somewhat negative distance, i.e., compression) a marked improvement in these parameters is found compared to available speakers. In particular, there is found a radical increase in vertical dispersion of the higher frequencies (i.e., greater than 3000 Hertz) as the spacing is decreased to touching.
In accordance with the principles of the invention, the reflecting surface could be a plane surface, a parabolic surface focused at a point within the prescribed distance, or a second dome-type speaker, the two speakers being driven in phase with each other and oriented so that their longitudinal axes are colinear. A speaker system as above described results in sound waves propagated from the first dome speaker interacting with sound waves coming from the second surface so as to produce a 360 degree, in phase propagation pattern normal to the longitudinal axis of the first speaker. The speakers could be oriented so that their longitudinal axes are not colinear, thereby providing a system having directional propagation characteristics while still maintaining the advantages of in-phase propagation.
According to another aspect of the invention, an audio speaker system comprises a first speaker having a forward sound propagating surface and a second speaker having a forward sound propagating surface, each speaker being oriented so that their forward surfaces face each other, and operating means for driving the first and second speakers simultaneously. The speakers are placed sufficiently close to each other so that sound energy propagated from one interacts with sound energy propagated from the other, thereby providing 360 degree, in-phase propagation. If the speakers are mounted so that their longitudinal axes are colinear, then the 360 degree sound propagation will be substantially normal to the longitudinal axes. In a specific embodiment, two dome-type speakers, which could be either mid-range speakers or high frequency speakers (tweeters), are oriented so that the domes are facing each other, and touching or spaced from each other no greater than two inches in the case of the mid-range and one inch in the case of the tweeter. More specifically, a distance from touching to two inches in the general range, from a frequency of 250 Hertz to 20,000 Hertz, whereas for speakers operating in the range of 1000-20,000 Hertz, the distance is preferably from touching to one inch.
The two speakers are driven in phase with each other so that sound waves from each act as an active reflector for sound waves of the other. The alternate compressing and rarefying of air contained between the two closely adjacent speakers provides an in-phase, 360 degree outwardly propagating sound wave having clarity and imaging characteristics heretofore unobtainable in conventional speaker systems.
According to a further embodiment of the invention, an audio speaker system, which in the below-described embodiment has been found to be particularly useful with respect to a bass speaker system, utilizes a casing having a first and second aperture. A first speaker having front and rear sound propagating surfaces is positioned internal to the casing so that its front surface faces outwardly through the first aperture. It is mounted in sealing contact with the first aperture, its rear surface being in reactive contact with an air chamber partially defined by the casing. A second speaker also having front and rear sound propagating surfaces is positioned over the second aperture so that its front surface faces into the air chamber and is in reactive contact therewith, its rear surface facing outwardly from the casing. The two speakers can be mounted so that their longitudinal axes are colinear. The casing and the two speakers define a substantially air-tight chamber. The first speaker and second speaker are driven by the same source, each being driven 180 degrees out-of-phase with respect to the other. The case can be spaced apart from a floor so that the first speaker is directed downwardly, thereby resulting in propagation from the rear surface of the second speaker having a slightly broader frequency propagation spectrum than that of the first speaker floor reflected spectrum due to selective higher frequency absorption by most floor surfaces. This reflected sound combines with sound propagated from the rear face of the second loud-speaker, thereby providing a 360 degree propagation system. The two speakers can be woofers, and when oriented according to the invention as above-described, provide 360 degree propagation of sound between 200 Hz and 600 Hz, frequencies below 200 Hz being generally considered non-directional. Since the air chamber is substantially air-tight, one can appreciate that a 180 degree phase mismatch between the two speakers will result in air within the chamber being alternately compressed and rarefied, each speaker extending inwardly into the chamber at the same time and extending outwardly from the chamber at the same time. It is this constant compression and rarefaction that prevents resonances and speaker over-excursions frequently experienced by conventional speaker systems. The 180 degree phase mismatch also results in back emf generated by each speaker canceling that of the other speaker at the driving source, thereby eliminating undesirable reflections from entering the driving source. The 360 degree propagation of this speaker system combined with the tendency of the closed air chamber to prevent unwanted speaker excursions, provides a realism heretofore unobtainable by conventional speaker systems.
In a further aspect of the invention, the two woofers as previously described are combined in a speaker system containing two mid-range dome-type speakers oriented as above-described and two tweeter dome-type speakers also oriented as above described, so that the longitudinal axes of all speakers are colinear with respect to each other. This speaker system provides 360 degree, in-phase sound propagation having clarity and imaging characteristics heretofore unobtainable by conventional systems.